The traditional reliability of telecommunication systems that users have come to expect and rely upon is based, in part, on the reliance on redundant equipment and power supplies. Telecommunication switching systems, for example, route tens of thousands of calls per second. The failure of such systems, due to either equipment breakdown or loss of power, is unacceptable, since such failure may result in the discontinuation of millions of telephone calls and a corresponding loss of revenue.
Power plants, such as battery plants, address the power loss problem by providing the system with an energy reserve (e.g., a battery) in the event of the loss of primary power to the system. A battery plant generally operates as follows. The battery plant includes a number of batteries, rectifiers and other power distribution equipment. The primary power is produced by the rectifiers, which convert an AC main voltage into a DC voltage to power the load equipment and to charge the batteries. The primary power may, however, become unavailable due to an AC power outage or the failure of one or more of the rectifiers. In either case, the batteries then provide power to the load. Redundant rectifiers and batteries may be added to the battery plant as needed to increase the availability of the battery plant.
A battery plant that powers telecommunications systems, such as transmission and switching systems in wireless base stations, commonly employs valve-regulated lead-acid (VRLA) batteries as the energy reserve. The batteries are typically coupled directly to the output of the rectifiers and may instantly provide power to the load in the event an AC power outage occurs. During normal operation, the batteries are usually maintained in a fully charged state to maximize a duration for which the batteries can provide energy to the load equipment.
As a battery ages, however, its capacity or energy-storage capability decreases thereby reducing a duration for which the battery can provide energy, even when fully charged. In many telecommunications applications, the battery is considered to have failed when its actual capacity has fallen below a threshold, such as 80% of its rated capacity. A failed battery should be replaced in an orderly fashion to maintain the availability of the battery plant. It is crucial, therefore, to be able to assess whether the capacity of a particular battery has fallen below it's threshold.
The capacity of a battery may be assessed with the battery on-line or off-line. One straightforward approach is to take the battery off-line and couple it to a dissipative-resistive load. The load can then discharge the battery completely or partially at a constant current, thus providing an indication of the battery's capacity. The off-line method, however, requires that the battery be temporarily removed from the battery plant, decreasing the availability thereof. Therefore, to maintain the battery plant at the desired availability level, the capacity of the battery should be assessed on-line.
Completely discharging the battery to assess the capacity also presents major disadvantages. If an AC power outage occurs during or after the discharge test, but before the battery has been fully recharged, the full energy reserve provided by the battery will not be available. This obviously jeopardizes the availability of the battery plant and the reliability of the telecommunications system powered therefrom. Further, since a battery may only be charged and discharged a finite number of times, each cycle of complete discharge and charge necessarily reduces the overall life span of the battery. Additionally, power is typically wasted and unwanted heat is usually generated through the use of conventional battery capacity testing.
Accordingly, what is needed in the art is a system and method for assessing the capacity of a battery that provides an accurate measurement of the battery's capacity while avoiding additional power loss and heat generation.